130ff8cb6c
Remove cgu_reuse_tracker from Session This removes a bit of global mutable state. It will now miss post-lto cgu reuse when ThinLTO determines that a cgu doesn't get changed, but there weren't any tests for this anyway and a test for it would be fragile to the exact implementation of ThinLTO in LLVM.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.